The Excited State Intramolecular Proton Transfer (ESIPT) reactions of dually fluorescent 2,5-bis(5-ethyl-2-benzoxazolyl)-hydroquinone (DE-BBHQ) and its isotopomers have been studied in the supersonic jet applying laser induced fluorescence (LIF) and fluorescence-depletion (F-D) spectroscopy. LIF-spectra measured at photo-tautomeric (red) fluorescence exhibit a characteristic triplet pattern of vibronic bands, which gradually collapses upon successive deuteration. Complementary TDDFT calculations indicate the possibility of 2 consecutive ESIPT reactions yielding an excited state diketo-tautomer. However, concerning this matter the present experimental results are not unambiguous and could be also rationalized without assuming the formation of an additional photo-tautomer. (C) 2015 Elsevier B.V. All rights reserved.